High efficiency operation is an important requirement in many switched reluctance machine (SRM) applications. Efficiency is increased by enhancing torque generation for an excitation. Related-art SRMs have reached an upper limit in the efficiency achievable through their shapes and configurations, but have not achieved the high power-densities provided by synchronous machines with high-energy-density permanent magnets.
FIG. 1 illustrates an SRM having common poles that have no excitation windings. SRM 100 includes a stator 102, a rotor 104, and a rotor shaft 106 that rotates rotor 104 within stator 102. Stator 102 has back iron 108 and salient excitation poles 110, 112 that each have a winding 114 through which an excitation current flows during an excitation phase associated with the excitation pole; excitation poles 110 are associated with phase A excitation, and excitation poles 112 are associated with phase B excitation. Stator 102 also has common poles 120 that have no excitation windings. Rotor 104 has back iron 116 and salient poles 118; rotor poles 118 may each be shaped (i.e., contoured) to provide a varying air gap as the rotor pole rotates past a stator pole or may be unshaped so as to provide a constant air gap with the stator pole as the rotor pole rotates past the stator pole. SRM 100 provides high power-density compared to an SRM having the same number of rotor poles and excitation poles, but no common poles.
Common poles 120 are disposed between excitation poles 110, 112 of different phases so as to prevent flux reversal within stator 102. The pole arc of each common pole 120 equals one rotor pole pitch, which is the angular distance between two adjacent rotor poles; this common-pole arc enables the equivalent of one rotor pole to be fully under the common pole at all times. Although each of two rotor poles may be partially under a common pole at a particular moment, the combined area of the rotor pole faces under the common pole remains constant throughout the rotation of rotor 104 and this combined area is equal to the area of a single rotor pole face.
The variation of reluctance between a rotor pole and an excitation pole increases as the rotor pole moves toward the excitation pole. But the reluctance variation between a common pole and a rotor pole is small and almost insignificant compared to the reluctance variation experienced by an excitation pole as the rotor rotates. Thus, near-constant reluctance is presented to a common pole and negligible reluctance variation is contributed by common poles 120 to SRM 100's overall reluctance variation. And because common poles 120 provide negligible reluctance variation, they do not appreciably contribute to torque generation; the machine torque comes almost entirely from the reluctance variation between the excited stator poles and their corresponding rotor poles.
Common poles 120: (1) provide a path for the flow of return flux, (2) always carry unidirectional flux, and (3) cause a unidirectional flow of flux in stator back iron 108, making the entire stator structure free of flux reversals. The absence of flux reversal minimizes core losses in SRM 100, thus boosting the efficiency and, indirectly, the power density of SRM 100.